1. Field of the Invention
Embodiments described herein relate generally to magnetic resonance imaging.
2. Description of the Related Art
MRI is an imaging method which magnetically excites nuclear spins of an object (a patient) set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation. The aforementioned MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance signal.
An MRI apparatus has a gradient magnetic field coil that applies a gradient magnetic field to an imaging space in which an object is placed, thereby providing an MR signal with spatial positional information, in its gantry. The gradient magnetic field coil generates significant heat during imaging because of a pulse current repeatedly supplied thereto. As the temperature of the gradient magnetic field coil rises, the magnetic field in the imaging region varies, and the center frequency of the magnetic resonance of hydrogen atoms in the object also varies, as a result.
Japanese Patent Laid-Open No. 2005-288025 discloses a known conventional technique relating to detection of the variation of the resonance frequency.
According to the Japanese Patent Laid-Open No. 2005-288025, variation of the resonance frequency in the imaging cross section is estimated based on the variation of a temperature measured by a temperature sensor disposed in the gantry. Then, the frequency of a reference clock of a control system is modified to follow the variation of the resonance frequency, and respective units are controlled based on the modified reference clock, thereby reduction of the influence of the disturbance in the static magnetic field homogeneity is attempted.
The center frequency of an RF pulse, such as a fat suppression prepulse and a 90° excitation pulse, is set based on the Larmor frequency of hydrogen atoms that depends on the intensity of the static magnetic field during an imaging preparation step such as a prescan. However, if the center frequency of the magnetic resonance of the hydrogen atoms shifts because of the heat generation of the gradient magnetic field coil during imaging after the conditions concerning RF pulses is set in the imaging preparation step, it becomes a contributory factor of degrading image quality.
For example, in imaging which takes a long time such as dynamic imaging, images whose data are acquired at later points in time suffer influence of a lager shift of the center frequency of the magnetic resonance due to the heat generation of the gradient magnetic field coil. Accordingly, in imaging which takes a long time, image quality of images whose data are acquired at later points in time may be more degraded because of the degradation of the effect of the fat suppression prepulse.
Therefore, a novel technology to obtain a high-quality image regardless of shift of a center frequency of magnetic resonance of hydrogen atoms caused by heat generation of a gradient magnetic field coil has been desired in MRI.